The present invention generally relates to material ablation with pulsed light sources, and particularly relates to focus depth control in a laser milling system.
Material ablation by pulsed light sources has been studied since the invention of the laser. Reports in 1982 of polymers having been etched by ultraviolet (UV) excimer laser radiation stimulated widespread investigations of the process for micromachining. Since then, scientific and industrial research in this field has proliferatedxe2x80x94mostly spurred by the remarkably small features that can be drilled, milled, and replicated through the use of lasers.
Ultrafast lasers generate intense laser pulses with durations from roughly 10xe2x88x9211 seconds (10 picoseconds) to 10xe2x88x9214 seconds (10 femtoseconds). Short pulse lasers generate intense laser pulses with durations from roughly 10xe2x88x9210 seconds (100 picoseconds) to 10xe2x88x9211 seconds (10 picoseconds). A wide variety of potential applications for ultrafast and short pulse lasers in medicine, chemistry, and communications are being developed and implemented. These lasers are also a useful tool for milling or drilling holes in a wide range of materials. Hole sizes as small as a few microns, even sub-microns, can readily be drilled. High aspect ratio holes can be drilled in hard materials, such as cooling channels in turbine blades, nozzles in ink-jet printers, or via holes in printed circuit boards.
Ultrafast and short pulse laser systems can be designed to accommodate drilling and milling of thin materials (50 to 100 microns); however, there is a need in the field of laser micromachining to drill or mill a wider range of material thicknesses. Ablating thicker materials presents certain technological challenges, including maintaining control of the laser through the entire thickness of the material and providing a finished product that meets both customer specifications and established quality standards.
Several problems continue to exist in the field of material ablation with pulsed light sources, and one such problem relates to controlling the quality specifications of a laser-drilled final product. Recent advancements in the field of laser drilling have been effective in enhancing the quality (i.e., shape, contour, and repeatability) of finished products according to customer demands. However, a growing number of customers require an increased geometric complexity in their product designs. Using conventional industry techniques, it is difficult to meet the increasing quality market needs. What is needed is a way to control the quality specifications of a laser-drilled final product.
Another problem that persists in the field of material ablation with pulsed light sources relates to controlling laser drilling on a thick workpiece. In typical laser-drilling applications that work with thin materials, ablation need only be controlled through the range of a thin workpiece (such as 50 to 100 microns). However, as material thickness increases, ablation must be controlled through a wider range of thickness and through an increased number of ablation layers. As drilling is performed on thicker surfaces, maintaining laser parameters becomes increasingly more difficult. Therefore, greater control over the laser parameters that control ablation rates and hole shape geometry throughout the process becomes necessary. What is needed is a way to control laser drilling on a thick workpiece.
A further problem that continues to exist in the field of material ablation with pulsed light sources relates to maintaining a constant ablation rate on the surface of a thick workpiece throughout the drilling process. During ablation of a layer in laser drilling, a material void is created at the contact point on the workpiece surface, i.e., the point at which the laser spot size intersects with the material surface. Once a void in the material is created, the expected contact point is no longer the same in that location on the workpiece due to the removal of material. Because of the variations in laser beam intensity and spot size at the contact point, the ablation rate changes. What is needed is a way to maintain a constant ablation rate on the surface of a thick workpiece throughout the drilling process.
A still further problem that continues to exist in the field of material ablation with pulsed light sources relates to maintaining constant laser beam intensity and constant spot size on the surface of a thick workpiece throughout the drilling process. Laser beam spot size is measured at the point where the focused laser beam and the surface of the workpiece material intersect. At a known laser beam intensity and known spot size, the ablation rate can be calculated and thus predicted in order to meet customer specifications. In a thick workpiece, however, as layers of material are ablated away the workpiece surface is no longer in the focal plane of the laser drilling system. This means that the laser beam intensity and spot size are no longer the same as originally intended (at the point of ablation) and, thus, ablation control is decreased. A constant spot size is required to prevent hole geometry distortion and inconsistent geometrical shaping due to lack of ablation control. What is needed is a way to maintain constant laser beam intensity and constant spot size on the surface of a thick workpiece throughout the drilling process.
According to the present invention, a method of adjusting depth of focus in a laser milling system includes generating a laser beam having a focal plane, positioning a workpiece in the focal plane, wherein a surface of the workpiece is exposed to the laser beam at a point intersecting the focal plane, and adjusting a position of at least one of the workpiece and the focal plane, thereby maintaining a constant ablation rate on the exposed surface of the workpiece throughout the drilling process.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.